One example of nanolithographic printing is Dip—Pen Nanolithography (“DPN”) printing. DPN printing is an ultrahigh-resolution direct-write lithography technique based upon the transport of chemically mobile materials (also referred to herein as “ink”) from a probe (or array thereof), usually a sharp tip, to a surface of interest (i.e., the “paper”) the probe contacts. For example, DPN printing allows one to draw patterns as small as one molecule high and a few dozen molecules wide.
By way of example of DPN printing methodology, octadecanethiol (ODT) is applied to an Atomic Force Microscope cantilever. When its tip is brought into contact with a gold substrate (and optionally rastered across it), the ODT molecules are transferred to and self-assemble on the gold surface, forming nanopatterns.
Current instrumentation capable of DPN printing include, for example, scanning probe microscope (SPM) technologies. Scanning probe microscopes (SPMs) scan a probe or array of probes over a sample surface and make local measurements of the properties of the sample surface. SPM can obtain detailed analyses of the topographical or other features of a surface, with sensitivities extending down to the scale of individual atoms and molecules. Several components are common to most SPMs: In addition to (a) high-precision (piezoelectric) scanners, an important component of the microscope is a tiny probe positioned in very close proximity to or in contact with a sample substrate surface and providing a measurement of its topography or some other physical parameter. The resolution is determined primarily by the shape of the tip and its proximity to the surface.
In an atomic force microscope (AFM), also called surface force microscopy, the probe generally includes a tip which projects from the end of a cantilever. Typically, the tip is very sharp to achieve maximum lateral resolution by confining the interaction to the end of the tip. By measuring motion, position or angle of the free end of the cantilever, many properties of a surface may be determined including surface topography, local adhesion, friction, elasticity, the presence of magnetic or electric fields, etc. In operation, an AFM typically will scan the tip of the probe over the sample while keeping the force of the tip on the surface constant, such as by moving either the base of the lever or the sample upward or downward to maintain deflection of the lever portion of the probe constant. Therefore, the topography of a sample may be obtained from data on such vertical motion to construct three-dimensional images of the surface topography. It is also known that AFMs can utilize analog and digital feedback circuits to vary the height of the tip of the probe or the sample based upon the deflection of the lever portion of the probe as an input. An image may be formed by scanning a sample with respect to the probe in a raster pattern, recording data at successive points in the scan, and displaying the data on a video display. The development of atomic/scanning force microscopy is described, for example, in articles by G. Binnig at al., Europhys. Lett., Vol. 3, p. 1281 (1987), and T. R. Albrecht et al., J. Vac. Sci. Technology, A6, p. 271 (1988). The development of the cantilever for AFMs is described, for example, in an article by T. R. Albrecht at al., entitled “Microfabricated Cantilever Stylus for Atomic Force Microscopy”. J. Vac. Sci. Technol., A8, p. 3386 (1990).
Other types of SPMs, such as scanning capacitance or scanning magnetic force microscopes, also use similar deflection sensors. Moreover, scanning tunneling microscope (STM) is generally similar to an SFM in overall structure and purpose, except that the probe comprises a sharpened conductive needle-like tip rather than a cantilever. The surface to be mapped is conductive or semiconductive. The metallic needle is typically positioned a few Angstroms above the surface. When a bias voltage is applied between the tip and the sample, a tunneling current flows between the tip and the surface. The tunneling current is exponentially sensitive to the spacing between the tip and the surface and thus provides a representation of the spacing. The variations in the tunneling current in an STM are therefore analogous to the deflection of the cantilever in an SFM. The head contains circuitry for biasing the tip with respect to the sample and preamplifying the tunneling current before it is passed to a controller.
Further background information can be found in the following: In U.S. patent publication 2002/0063212 A1 to Mirkin et al. (published May 30, 2002); PCT publication WO 01/91855 A1; and S. Cruchon-Dupeyrat et al., Applied Surface Science, 175–176 (2001), 636–642.
In DPN printing, multiple parameters can affect (1) the rate of transport of the ink from the tip to the substrate; (2) its spreading on the substrate, among other factors, and hence (3) the geometric dimensions (and other characteristics) of the fabricated patterns. These parameters include the following:                Instrument control parameters, including the scan speed and the force applied by the tip,        Geometric factors, including the sharpness (apex radius) of the probe tip, the roughness and grain size of the substrate,        Physicochemical factors, including the type of the substrate, ink, tip and ambient medium. Examples are the chemical composition of the ink, the ink preparation and tip coating methodology, the surface chemistry of the tip and substrate, the temperature and the relative humidity (when used in air or gaseous medium).        
The number of influential parameters is large; some of the parameters can be difficult or expensive to measure or to control, or they may fluctuate from run to run or during the fabrication procedure. For example, the ambient temperature and humidity are of particular importance. In order to fabricate the desired nanopatterns with the proper characteristics (e.g. size), the instrumental parameters need to be adjusted as a function of the factors that vary.
The practitioner, therefore, needs calibration procedures that will easily, if possibly automatically, determine the relationship between one or more of the parameters cited above and one or more of the (geometric) characteristics of the pattern.